The Doctor agrees to transmat down to Earth to make sure everything is
okay before the Nerva survivors begin to reclaim their planet. There, he,
Sarah Jane and Harry discover the presence of a Sontaran named Styre, who
is performing cruel experiments on a band of captive humans. Styre's goal
is to discover the weaknesses of the human body -- weaknesses the
Sontarans will then exploit in their quest to dominate the universe.

Production

Since 1971, a Doctor Who season had been comprised almost
exclusively of four- and six-part stories. As he developed the outline of
the programme's twelfth recording block in early 1974, however, script
editor Robert Holmes decided to try a different approach. He was not a
fan of lengthy stories, believing that most such adventures were
excessively padded. So, as an experiment, Holmes elected to replace what
would normally be a six-part story with two serials -- one four episodes
long, the other two episodes long.

In so doing, Holmes had to be careful not to put additional strain on
Doctor Who's schedule and budget. Rather than assign both location
and studio time to each of the two new adventures, Holmes decided that one
would be made entirely on location, while the other would be confined
exclusively to the studio. In this manner, the two serials could
effectively function as a single production, just like the six-part story
they were supplanting.

Although Robert Holmes was averse to old monsters, the
Sontarans' return was a way to capitalise on the existing costume

Already commissioned at this stage was a suitable four-part adventure:
“Space Station” by Christopher Langley. For the shorter story,
Holmes approached the writing team of Bob Baker and Dave Martin; the duo
had most recently handled The Three Doctors
two years earlier. Holmes gave Baker and Martin a general idea of the
storyline he wanted, including its setting of a far-future, abandoned
Earth and the return of the Sontarans, a race Holmes himself had
introduced in The Time Warrior the previous
year. Although Holmes was generally averse to bringing back old monsters,
he saw this as a way to capitalise on the cost of the Sontaran costume and
the spaceship exterior prop fabricated for the earlier adventure; hence
Baker and Martin were told to write with the constraint that only one
Sontaran costume would be available. (The authors gave their Sontaran the
name Weam Styre, although in the event he would only be referred to as
Styre.)

Baker and Martin were commissioned to write a storyline for “The
Destructors” on May 23rd. The script for the first episode was
requested very shortly thereafter, on June 5th, while the commission for
part two following on July 6th. By this time, “Space Station”
had been abandoned, and “The Destructors” was paired with a
new story called The Ark In Space.

From the start, Baker and Martin envisioned filming their serial in the
West Country. Initially, however, they wanted to incorporate some relics
of human civilisation which would imply that “The Destructors”
was actually set where London had once stood -- for instance, by having
the top of Nelson's column poking up out of the ground in homage to the
1968 post-apocalyptic film Planet Of The Apes. Much of the action
of their scripts was set in the ruins of an old priory, and so many of
Styre's torture devices were medieval in nature. Holmes rewrote episode
two extensively, in the process removing a subplot in which it would have
been revealed that Styre was controlling Vural's mind.

Brought on board to direct both The Ark In
Space and “The Destructors” was Rodney Bennett.
Bennett had begun his career as a producer for radio before shifting to
television when BBC2 was created. He went on to direct episodes of
programmes such as Z Cars and Thirty Minute Theatre. At the
suggestion of production unit manager George Gallaccio, Bennett decided to
record “The Destructors” using the BBC's new Outside
Broadcast (OB) videotape facilities rather than the traditional film, in
order to make editing more convenient. However, a limitation of OB work
was that colour separation overlay -- an effect often called into
service on Doctor Who -- could not be employed. Styre's robot had
originally been envisioned as a CSO creation, but it would instead have
to be realised as a practical prop.

The writers were unhappy with the title The Sontaran Experiment, because they felt it
ruined their only cliffhanger

Around September 18th, Holmes and producer Philip Hinchcliffe decided to
change the title of the story -- which been given the production code
Serial 4B -- to The Sontaran Experiment. This disappointed Baker
and Martin, who felt that it ruined their only cliffhanger. Ironically,
despite Holmes' intentions, a new, more lightweight costume was created
for Kevin Lindsay to wear as Styre. Lindsay had also played Linx in The Time Warrior, and had found the costume
very demanding to wear. (This was exacerbated by Lindsay's heart
condition, an ailment which would result in his death just six months
later.)

Although Bennett concurred with Baker and Martin's suggestion that The
Sontaran Experiment should be recorded in the West Country, scouting
revealed that no suitable site could be located for the authors' intended
priory setting. Holmes therefore rewrote the action to take advantage of
Dartmoor's natural rock formations. Taping began with three days near
Postbridge from September 26th to 28th, for scenes set around the
transmat, the humans' camp, and the pit. Amongst the guest cast was Glyn
Jones, who had previously been involved with Doctor Who in a
completely different capacity, writing The Space
Museum in 1965.

Recording then shifted to Hound Tor near Manaton beginning on the 29th,
the locale for material in the vicinity of Styre's craft. Late on the
first day, however, Tom Baker slipped on a patch of wet grass while
enacting the Doctor's confrontation with Styre before a captive Sarah.
Recording stopped (forcing the abandonment of a sequence in which the
Doctor and Harry meet and hear Sarah Jane screaming) and Baker had to be
carried back to the nearest car and driven to the hospital. There, it was
discovered that he had cracked his collarbone. Nonetheless, despite his
discomfort, Baker was able to finish shooting The Sontaran
Experiment. He was able to conceal a neck brace beneath his scarf and
coat, although his performance was restricted as he was forced to remain
very still. Work at Hound Tor continued until October 2nd, marking the
conclusion of work on Doctor Who's first two-part story since The Rescue in 1964.